The present invention relates to an apparatus to coat coating solution in an extremely uniform layer thickness on a web (which is a belt-like flexible support) while the web is supported floatingly.
More particularly, it relates to an apparatus for running a web continuously while supporting and floating a surface of the web having been coated with a photographic light-sensitive material and coating one or more kinds of coating solution onto the reverse side of the web opposite to the coated surface thereof. Specifically the invention relates to a web coating apparatus appropriate to perform both-sided coating continuously.
Conventionally, as a technology for both sided coating, various devices and methods are known, including the following examples:
(1) A method to coat one side of a web, make the coated surface of the web gelled, and then successively coat on the opposite side thereof, while bringing the gelled side in contact with the support roll directly. (Japanese Patent Examined Publication No. 44171/1973)
(2) A method wherein the web is floated up by an air jet from a curved surface of a roll having therein small holes or slits and the edge of the coater is pressed to the web so as to coat it. (Japanese Patent Examined Publication No. 17853/1974)
But the above-mentioned conventional technologies have the following defects:
(1) Even when there are fine dust or scratches on the surface of a support roll that supports a gelled surface, the gelled coated surface is disturbed. When a part of the coated layer adheres on the roll, same trouble is generated. Therefore, maintenance for the support roll is very difficult.
(2) Even when the circumferential speed of the support roll deviates from the conveying speed for the web slightly, gelled coated layer is disarranged remarkably.
Besides, the technology opened in Japanese Patent Examined Publication No. 17853/1974 has the following disadvantageous points:
(3) When the width of a web is large, the difference of the floating level in the width-wise direction becomes large. Therefore, the edge of the coater cannot be pressed to the web uniformly, which makes it difficult to get uniform coating through the whole surface.
(4) Since no consideration is taken to suppress the vibration of a web before and after the coater in this technology, uneven coating easily takes place.
(5) Due to a method to press the coater to a web, a weakness arises with bead coating methods, such as the slide hopper method and others which are normally used in the coating for photographic light-sensitive materials, cannot be used.
So, the present inventors have proposed and put to practical use a web coating method and its apparatus wherein a coater and an air jetting device both arranged to face each other with a continuously running web between them, and the jetting device jets air to the web to support it floatingly and contactlessly from the roll. (Japanese Patent No. 175801/1981)
The first problem of the conventional web coating apparatus was that there had been a possibility of troubles caused by the evaporation of solvent from the web surface in time of successive both sided coating onto a web.
That is to say, in the both sided coating process, a web is kept floating by the air jetting device and conveyed under the condition that coating layers containing almost gelled emulsion stay on the web surface (facing the air jetting device). And, the coating solution whose temperature is from 25.degree. C. to 40.degree. C. is coated onto the reverse side of the web (coater side) by the coater. Due to the coating of high temperature coating solution like this, the temperature of the web is increased. Therefore, the evaporation of solvents (such as vapor) from the coating layers on the web surface is promoted, and since the gap between the web and the air jetting device is so narrow (normally, not more than a few mm), the evaporating vapor becomes saturated in the gap space. Further, with this condition, if the surface temperature of the air jetting device and the gap space atmospheric temperature are lower than the temperature of the saturated vapor, the vapor is condensed again on the surface of the air jetting device to generate dew drops condensation, which attach to the web surface again to finally cause damage on the web surface.
Considering the above-mentioned points, the first object of the present invention is to provide a web coating apparatus wherein vapor from the coating layers on web surface in the air jetting side has no possibility to be condensed again in the gap space.
A second problem in the conventional web coating apparatus, as shown in FIG. 8, results when the web floating level is lowered to form the web deforming part 3c so as to bring the part in contact with the web holding surface at the border area and the inlet and outlet sides of the vacuum area on the web holding surface 11a of the air jetting device 11 corresponding to the vacuum chamber 8.
That is to say, a web supported floatingly by the air jetting device 11 runs normally around the roll, keeping at least a certain distance from the web holding surface 11a. In the border area at the inlet and outlet sides of the vacuum area on the web holding surface corresponding to the vacuum chamber 8, however, the back pressure (a static pressure of the space between the web and web holding surface) at the vacuum area corresponding to the vacuum chamber becomes lower than that of other areas. So, a phenomenon wherein high static pressure built up between the web and the web holding surface flows into the vacuum area is generated. As a result, an amount of high static pressure gas staying at the border area is decreased rapidly to cause the web to float downward. Thus, the web contacts the web holding surface.
Considering the above-mentioned point, the second object of the present invention is to provide a web coating apparatus which can prevent the aforesaid contact trouble by keeping the floating level of a web at a certain height or more in the border area.
Next, the third problem of the above-mentioned conventional coating apparatus is described.
In the above-mentioned conventional technology, a vacuum chamber is used for stabilizing the bead (which is a coating solution bridged on the web surface from the coater.) by sucking it down. Air current in the vacuum chamber gives influence on the coating condition to produce coating unevenness. This occurs because the relation between the movement of the air current and the coating unevenness in the vacuum chamber was not clear, as well as that there was no means to control or adjust the air current.
The present inventors discovered, after an accurate analysis of the air current conditions, the reason for the coating unevenness is because air current flowing in from the web inlet of the vacuum chamber does not defuse entirely but reaches the bead gap portion to make the bead unstable.
Taking the above-mentioned points into consideration, the third object of the present invention is to provide a web coating apparatus wherein air current flowing into the vacuum chamber does not exert any influence on the bead gap portion.